This invention relates to a method for the manufacture of silicon carbide and more particularly to a method for the manufacture of finely divided high-purity silicon carbide suitable for use as a raw material for the production of high-strength sintered articles of silicon carbide.
Silicon carbide has excellent thermal resistance and high-temperature strength. Silicon carbide in a sintered form, therefore, is expected to find utility in various high-temperature structural members such as high-temperature gas turbines, for example. The thermal and mechanical properties of the sintered articles of silicon carbide depend heavily on the attributes of silicon carbide powder, the raw material for the sintered articles. To achieve the optimum effect, silicon carbide powder is desired to be of high-purity possessing as small a particle diameter as possible and not to suffer from dispersion of particle shape and size. Among the methods developed and adopted to date for the synthesis of silicon carbide, those widely known in the art are:
(i) reaction of metallic silicon and carbon, PA1 (ii) reaction of silicon tetrachloride, hydrogen and carbon, PA1 (iii) reaction of silica (SiO.sub.2) and carbon.
Of these methods, the method (iii) enjoys a number of commercial advantages such as low cost of raw materials used, simplicity and ease of reactions and attendant operations involved in the synthesis, and freedom from use of raw materials capable of corroding the equipment in use. However, it is difficult to obtain fine powder of silicon carbide by the method of (iii).
Several improved versions of the method of (iii) have been proposed with a view to materializing production of finely divided silicon carbide. One trend of these improved versions is toward using a finely divided carbon powder and a finely divided SiO.sub.2 powder as the raw materials thereby producing finely divided silicon carbide powder. Generally in a raw material obtained by mixing different solid powders, the homogeneity of mixture and the intimacy of contact between the powders have their limits. In the present case, therefore, part of the SiO.sub.2 tends to remain in reaction products, frequently making it necessary to remove the SiO.sub.2 with hydrofluoric acid. Even when there is used a SiO.sub.2 powder having a very small particle diameter of 0.01.mu., for example, the produced silicon carbide powder has a particle soze on the order of 0.6.mu. (Japanese Unexamined Patent Publication No. 126699/1979).
In one known method, colloidal silica which has silica particles colloidally suspended in water is used as the SiO.sub.2. Japanese Patent Publication No. 34028/1970 (corresponding to British Pat. No. 1,099,647) cites quartz, sand, silicic acid, silica gel, amorphous silica, colloidal silica, and many others as examples of the SiO.sub.2 source but makes no mention whatever about the advantage derivable from the use of colloidal silica.
Hase and Suzuki, Journal of Ceramics Society of Japan, 86, 541 (1978) reports an observation that SiC found by X-ray analysis to possess an apparent crystallite size of 530 to 600 A was obtained by using colloidal silica. Also in this case, the product was treated with hydrofluoric acid. The particle size of the SiC by SEM is mentioned nowhere in this report.
Colloidal silica is a colloidal solution having a milky white color. When it is mixed with a solid carbon source, the homogeneity of mixture is higher than when SiO.sub.2 powder is mixed with a solid carbon source. The higher homogeneity of mixture is thought to have a better effect upon the reaction for the formation of SiC. No such observation is mentioned anywhere in the literature cited above.
The absence of such better effect may possibly be ascribed to the large size of individual silica particles in the colloidal silica. Even if the expected better effect is obtained at all, since the colloidal silica contains sodium or some other alkali metal compound as a stabilizer, it does not prove advantageous as a raw material for the production of silicon carbide powder of high purity.